Nitric oxide (NO) has been recognized as an unusual messenger molecule with many physiologic roles, in the cardiovascular, neurologic and immune systems (Griffith, T M et al., J Am Coll Cardiol, 1988, 12:797-806). It mediates blood vessel relaxation, neurotransmission and pathogen suppression. NO is produced from the guanidino nitrogen of L-arginine by NO Synthase (Moncada, S and Higgs, E A, Eur J Clin Invest, 1991, 21(4):361-374). In mammals, at least three isoenzymes of NO Synthase have been identified. Two, expressed in neurons (nNOS) and endothelial cells (Type III-ecNOS), are calcium-dependent, whereas the third is calcium-independent and is expressed by macrophages and other cells after induction with cytokines (Type II-iNOS) (Bredt, D S and Snyder, S H, Proc Natl Acad Sci USA, 1990, 87:682-685, Janssens, S P et al., J Biol Chem, 1992, 267:22964, Lyons, C R et al., J Biol Chem, 1992, 267:6370-6374). The various physiological and pathological effects of NO can be explained by its reactivity and different routes of formation and metabolism.
Recent studies suggest that a loss of endothelial-derived NO activity may contribute to the atherogenic process (O'Driscoll, G, et al., Circulation, 1997, 95:1126-1131). For example, endothelial-derived NO inhibits several components of the atherogenic process including monocyte adhesion to the endothelial surface (Tsao, P S et al., Circulation, 1994, 89:2176-2182), platelet aggregation (Radomski, M W, et al., Proc Natl Acad Sci USA, 1990, 87:5193-5197), vascular smooth muscle cell proliferation (Garg, U C and Hassid, A, J Clin Invest, 1989, 83:1774-1777), and vasoconstriction (Tanner, F C et al., Circulation, 1991, 83:2012-2020). In addition, NO can prevent oxidative modification of low-density lipoprotein (LDL) which is a major contributor to atherosclerosis, particularly in its oxidized form (Cox, D A and Cohen, M L, Pharm Rev, 1996, 48:3-19).
It has been shown in the prior art that hypoxia downregulates ecNOS expression and/or activity via decreases in both ecNOS gene transcription and mRNA stability (Liao, J K et al., J Clin Invest, 1995, 96:2661-2666, Shaul, P W et al., Am J Physiol, 1997, 272: L1005-L1012). Thus, ischemia-induced hypoxia may produce deleterious effects, in part, through decreases in ecNOS activity.
HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase is the microsomal enzyme that catalyzes the rate limiting reaction in cholesterol biosynthesis (HMG-CoA.fwdarw.Mevalonate). An HMG-CoA reductase inhibitor inhibits HMG-CoA reductase, and as a result inhibits the synthesis of cholesterol. A number of HMG-CoA reductase inhibitors has been used to treat individuals with hypercholesterolemia. Clinical trials with such compounds have shown great reductions of cholesterol levels in hypercholesterolemic patients. Moreover, it has been shown that a reduction in serum cholesterol levels is correlated with improved endothelium-dependent relaxations in atherosclerotic vessels (Treasure, C B et al., N Engl J Med, 1995, 332:481-487). Indeed, one of the earliest recognizable benefits after treatment with rho GTPase function inhibitors is the restoration of endothelium-dependent relaxations or ecNOS activity (supra, Anderson, T J et al., N Engl J Med, 1995, 332:488-493).
Although the mechanism by which HMG-CoA reductase inhibitors restore endothelial function is primarily attributed to the inhibition of hepatic HMG-CoA reductase and the subsequent lowering of serum cholesterol levels, little is known on whether inhibition of endothelial HMG-CoA reductase has additional beneficial effects on endothelial function.
By inhibiting L-mevalonate synthesis, HMG-CoA reductase inhibitors also prevent the synthesis of other important isoprenoid intermediates of the cholesterol biosynthetic pathway, such as farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP) (Goldstein, J L and Brown, M S, Nature, 1990, 343:425-430). The isoprenoids are important lipid attachments for the post-translational modification of variety of proteins, including G-protein and G-protein subunits, Heme-a, nuclear lamins, Ras, and Ras-like proteins, such as Rho, Rab, Rae, Ral or Rap (Goldstein, J L and Brown, M S, supra; Casey, P J, Science, 1995, 268:221-225). The role that isoprenoids play in regulating ecNOS expression, however, is not known.
Pulmonary hypertension is a major cause of morbidity and mortality in individuals exposed to hypoxic conditions (Scherrer, U et al., N Engl J Med, 1996, 334:624-629). Recent studies demonstrate that pulmonary arterial vessels from patients with pulmonary hypertension have impaired release of NO (Giaid, A and Saleh, D, N Engl J Med, 1995, 333:214-221, Shaul, P W, Am J Physiol, 1997, 272: L1005-L1012). Additionally, individuals with pulmonary hypertension demonstrate reduced levels of ecNOS expression in their pulmonary vessels and benefit clinically from inhalation nitric oxide therapy (Roberts, J D et al., N Engl J Med, 1997, 336:605-610, Kouyoumdjian, C et al., J Clin Invest, 1994, 94:578-584). Conversely, mutant mice lacking ecNOS gene or newborn lambs treated with the ecNOS inhibitor, Nw-monomethyl-L-arginine (LNMA), develop progressive elevation of pulmonary arterial pressures and resistance (Steudel, W et al., Circ Res, 1997, 81:34-41, Fineman, J R et al., J Clin Invest, 1994, 93:2675-2683). It has also been shown in the prior art that hypoxia causes pulmonary vasoconstriction via inhibition of endothelial cell nitric oxide synthase (ecNOS) expression and activity (Adnot, S et al., J Clin Invest, 1991, 87:155-162, Liao, J K et al., J Clin Invest, 1995, 96, 2661-2666). Hence, hypoxia-mediated downregulation of ecNOS may lead to the vasoconstrictive and structural changes associated with pulmonary hypertension.
Often cited as the third most frequent cause of death in the developed countries, stroke has been defined as the abrupt impairment of brain function caused by a variety of pathologic changes involving one or several intracranial or extracranial blood vessels. Approximately 80% of all strokes are ischemic strokes, resulting from restricted blood flow. Mutant mice lacking the gene for ecNOS are hypertensive (Huang, P L et al., Nature, 1995, 377:239-242, Steudel, W et al., Circ Res, 1997, 81:34-41) and develop greater intimal smooth muscle proliferation in response to cuff injury. Furthermore, occlusion of the middle cerebral artery results in 21% greater infarct size in "ecNOS knockout" mice compared to wildtype mice (Huang, Z et al., J Cereb Blood Flow Metab, 1996, 16:981-987). These findings suggest that the ecNOS production may play a role in cerebral infarct formation and sizes. Additionally, since most patients with ischemic strokes have average or normal cholesterol levels, little is known on what the potential benefits of HMG-CoA reductase inhibitor administration would be in cerebrovascular events.
There exists a need to identify agents that improve endothelial cell function.
There also exists a need to identify agents that can be used acutely or in a prophylactic manner to treat conditions that result from low levels of endothelial cell Nitric Oxide Synthase.